BGA Board Design, Manufacturing, and Storage Considerations

Printed Circuit Boards which incorporate Ball Grid Arrays (BGAs) must be designed and manufactured to prevent solder from migrating through vias (holes) during the soldering process. This is a vital consideration if the board will have parts on both sides and go through a wave soldering process.

Design Requirements

Design the solder mask artwork to tent (cover) all vias associated with BGA s. Design the solder mask to cover the largest areas possible. Avoid the use of delicate, narrow slivers of solder mask whenever possible.

Specify that the board house plug all vias associated with BGA s. The via plugs will prevent solder from wicking from one side of the board to the other. Without plugs, the solder balls can be pulled away from the BGA or solder can be pulled from the opposite side of the board and cause shorts beneath the BGA.

Specify that the board house apply the Solder Mask Over Bare Copper (SMOBC)

NOTE - before the board is plated. The mask acts as a dam to prevent the solder from flowing into areas where it is not wanted. Any plating beneath the mask will invite solder to flow under the mask and defeat (undermine) its purpose.

Board Construction Requirements

After lamination and drilling, raw printed circuit boards normally go through a pumice scrubber. The boards then go to the Screening Department. Boards which will be used with BGA s should be plugged and masked in the following order before screening and plating.

Plug all vias associated with the BGA using either of the following clear (un- pigmented) epoxy materials:

a. UV Curable

b. LPI (Liquid Photo Imagable)

The clear (un-pigmented) material facilitates uniform curing of the plug all the way through the board. If the center of the plug is not properly cured, it may out-gas during the soldering process, blow the plug out of the board, and damage the solder mask.

Apply solder mask over the plugged BGA vias. This process is called tenting . The solder mask must be applied over bare copper - before the board is plated. The mask acts as a dam to prevent the solder from flowing into areas where it is not wanted. Any plating beneath the mask will invite solder to flow under the mask and defeat its purpose.

The board is then ready for screening and plating.

Storage Requirements

Multi-layer Printed Circuit Boards and Ball Grid Arrays must be stored in sealed containers with desiccant (moisture absorbing material). Otherwise, moisture from the air will be absorbed by both.

Multi-layer Printed Circuit Boards which have been exposed to moisture should be baked prior to soldering in order to drive moisture out of the board. Trapped moisture will turn to steam during the soldering process and can cause the PC Board to delaminate.

BGA s are even more susceptible to absorbing moisture and should not be exposed to ambient air for more than a few hours. Unless they are properly baked, BGA s will popcorn (delaminate due to the pressure of internal steam) during the soldering process.

For in-depth information on this subject, please refer to the Joint Industry Standard IPC/JEDEC J-STD-033 entitled Standard for Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices.

This information is supplied as an overview for our customers. If this information is of interest, we recommend a more thorough study of the reference documents cited above. ACC Electronix, Inc. assumes no legal liability for the accuracy of the information contained herein or in the reference documents listed.